1. Field of the invention
This invention relates to systems receiving electromagnetic field signals using a coil as a pickup antenna. A major application is NMR and MRI systems where a high-Q coil is used to pick up the magnetic fields resulting from the precessing moments.
2. Description of Prior Art In some applications, principally magnetic resonance systems, a coil is used to pick up the electromagnetic field signals with the coil acting as an antenna. The coil resistance is often the major source of noise in the system and dominates the performance. To avoid this noise, some systems have used superconductive high-Q coils. Such a system is described in M. R. Bendall and D. T. Pegg, J. Magn. Res. 67, 376 (1986) and in R. Black et al., Science 259, 793 (1993).
The problem with this approach is that the high Q coil and its associated tuning capacitor results in a very narrow bandwidth, inadequate for typical magnetic resonance imaging systems. The bandwidth is particularly inadequate for high-speed MRI systems. As a result, these systems use various matching systems which compromise the SNR (signal to noise ratio). In an ideal system the body noise will dominate the coil noise. This is particularly difficult to achieve in systems which receive at a relatively low frequency. One example of MRI systems which receive at low frequencies are pulsed MRI systems which use a strong polarizing fields and a relatively low readout bias field. Systems of this type are described in U.S. Pat. No. 5,057,776 by the same inventor, in a paper "Novel Approaches to Low-Cost MRI", A. Macovski and S. Conolly, MRM 29:1-10 (1993) and in a paper by B. Favre, J.Bonche, H. Mehier, J. Peyrin, Magn. Reson. Med. 13, 299-304 (1990).
In the invention to be described, a high-Q coil is used as an antenna. To provide the desired bandwidth the coil is periodically de-energized with the signal allowed to build up. A somewhat related operation takes place in digital receiving systems which use matched filters. Here, in some cases, the matched filter is implemented using a shorted tuned circuit. Here, however, the coil is not used as an antenna and narrow bandwidth is not an issue. A system of this type is described in Digital Communications by Simon Haykin, Wiley 1988, pages 90-92. The coil is not the pickup antenna. A wideband digital signal is applied to the coil which is periodically shorted out to act as a matched filter.